tools: add litex_crossover to be able to use lxterm (and serialboot) over a crossover UART (bridged over UART/Ethernet/PCIe/USB, etc...).

This is still a proof of concept but can be used/tested with:
lxsim --with-etherbone --uart-name=crossover --csr-csv=csr.csv
lxserver --udp --udp-ip=192.168.1.51
lxcrossover (will indicate the virtual_tty)
lxterm virtual_tty

litex/tools/litex_crossover.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+
+# This file is Copyright (c) 2020 Florent Kermarrec <florent@enjoy-digital.fr>
+# License: BSD
+
+# Proof of Concept to use the crossover UART with lxterm over a bridge.
+
+import os
+import pty
+import threading
+
+from litex import RemoteClient
+
+wb = RemoteClient()
+wb.open()
+
+# # #
+
+def pty2crossover(m):
+    while True:
+        r = os.read(m, 1)
+        wb.regs.uart_xover_rxtx.write(ord(r))
+
+def crossover2pty(m):
+    while True:
+        if wb.regs.uart_xover_rxempty.read() == 0:
+            r = wb.regs.uart_xover_rxtx.read()
+            os.write(m, bytes(chr(r).encode("utf-8")))
+
+m, s = pty.openpty()
+print("LiteX Crossover UART created: {}".format(os.ttyname(s)))
+
+pty2crossover_thread = threading.Thread(target=pty2crossover, args=[m])
+pty2crossover_thread.start()
+
+crossover2pty(m)
+
+# # #
+
+wb.close()

setup.py

@@ -39,12 +39,14 @@ setup(
             "litex_term=litex.tools.litex_term:main",
             "litex_server=litex.tools.litex_server:main",
             "litex_sim=litex.tools.litex_sim:main",
+            "litex_crossover=litex.tools.litex_crossover:main"
             "litex_read_verilog=litex.tools.litex_read_verilog:main",
             "litex_simple=litex.boards.targets.simple:main",
             # short names
             "lxterm=litex.tools.litex_term:main",
             "lxserver=litex.tools.litex_server:main",
             "lxsim=litex.tools.litex_sim:main",
+            "lxcrossover=litex.tools.litex_crossover:main",
         ],
     },
 )